Carbon dioxide is used throughout the winemaking process to displace oxygen from contact with crushed grapes or wine. At some wineries, carbon dioxide is deliberately pumped over white grapes as they are received at the winery and pass through the destemmer in order to minimize oxidation. Draining tanks, presses, storage and blending vats, filters, and bottling lines are all locations where carbon dioxide may be applied by fastidious winemakers.
Carbon dioxide also plays an important role in the fermentation of all wines. Like humans, yeast metabolizes starches and sugars to produce water and carbon dioxide. In the human case, carbon dioxide from muscle or brain activity dissolves in the blood, is transferred to the lungs and then to the atmosphere as exhaled breath. In the case of yeast’s metabolic activity in six-carbon sugar solutions such as grape juice, the three main by-products are water, ethanol, and carbon dioxide. If excess oxygen is available, the yeast obtains more cell-building energy from the sugar by converting it to carbon dioxide and water. With only a moderate oxygen supply, the yeast produces carbon dioxide and the ethanol that distinguishes wine from grape juice.